1. The Field of the Invention
The present invention relates in general to an assembly which is loaded into a fast reactor for the purposes of transmutation treatment of a long-lived radioactive material. More particularly, the present invention relates to an assembly for transmutation of a long-lived radioactive material, in which the transmutation assembly is composed solely of FP (fission product)--containing pins, in which each of the FP pins has a cladding tube containing therein a moderator and a radioactive material including long-lived fission product (LLFP) nuclides in such a manner that the radioactive material is surrounded by, or in other words covered with, the moderator material in the cladding tube.
2. Description of Prior Art
Utilization of nuclear energy is inevitably followed by the generation, more or less, of long-lived fission products (LLFP). High level radioactive waste which has been disposed of by glassification generally contains long-lived radioactive FP nuclides, such as technetium-99 and iodine-129, which have a half-lives of about 210 thousand years and about 1.6 million years, respectively. Technetium-99 is water-soluble and there is fear that it is, after a long period of time, eluted and released out of a barrier in the form of ions of TcO.sub.4.sup.-, etc. following geologic disposal. With respect to iodine-99, considering its safety from the viewpoint of migration into the ground water when the iodine-99 is solidified and then subject to geologic disposal, there are the problems that this material has a high solubility and has a low absorption by the barrier. Thus, from a viewpoint of reduction of the environmental load, these long-lived FP nuclides should preferably be transformed into the other stable nuclides by a suitable method before a final disposal thereof. For example, the technetium-99 and iodine-129 are transformable, by a neutron absorption reaction, into non-radioactive and stable ruthenium (Ru) and xenon (Xe).
Technetium-99 and iodine-129, which are long-lived FP nuclides, have larger neutron absorption cross sections for thermal neutron energy than for fast neutron energy, and have larger resonance absorption regions in a lower energy region (approximately 5 eV, etc.). Therefore, in order to transmutate these long-lived FP nuclides by neutron absorption reactions, it is advisable to slow down to some extent fast neutrons which have been generated by nuclear fission, and then use the same as resonance energy neutrons or thermal neutrons.
Both thermal neutron reactors and fast neutron reactors are considered as reactors in which to transmute long-lived FP nuclides. A thermal neutron reactor utilizes moderated or slowed-down neutrons and, therefore, the nuclides can be transmuted, to some extent, by loading pins which contain technetium-99 or iodine-129 into the reactor core. A fast neutron reactor, on the other hand, requires the fast neutrons to be moderated and, therefore, has utilized an assembly having a structure in which long-lived FP nuclides-containing pins and moderator-containing pins are housed together in a wrapper tube, for the purpose of transmutation of these long-lived FP nuclides.
A conventional assembly for the purpose of transmutation is shown in FIG. 6, in which moderator-containing pins 12 and FP-containing pins 14 are disposed in a dispersed arrangement in a wrapper tube 10 as illustrated. The moderator pin 12 is composed of a cladding tube which contains moderator material and nothing else, and the FP pin 14 is composed of a cladding tube which contains the material including long-lived FP nuclides, and nothing else.
In the case where the assemblies for the transmutation purposes as described above are loaded into a blanket region of the fast reactor, the transmutation rate of the long-lived FP nuclides is approximately 2.0-2.5% which is not as much as the requirement. The inventor of the present invention considers that the reason for such a low transmutation rate resides in a remarkably high self-shielding effect of the neutrons in the FP pins such that the nuclear reactions between the FP and the neutrons are carried out only near the surface of the FP pins, with the result that the neutrons are prevented from entering deep into the FP pins. This problem would be solved to a limited extent by an attempt at forming thinner FP pins but this measure would raise other serious problems in that the necessary number of pins is increased, resulting in deficiencies in production, working effect and cost performance.
Further, in the conventional structure of the transmutation-purpose assemblies as described above, the arrangement of the FP pins and the moderator pins in the space of the assembly is complex, resulting in less working efficiency and difficulties in inspection of the pins.